In the human body the epidural space is a narrow but important anatomical space or void, filled with areola-type tissue, located in and traversing most of the length of the spine. The epidural space exhibits an internal pressure which research has determined to be at or below atmospheric pressure, customarily measured in units of centimeters of water. The epidural space is important because the epidural space may be used to introduce certain pharmaceutical agents at various positions along the spine. By introduction of anesthetics and/or medication into the epidural space at an appropriate position, a variety of pain causing maladies that afflict the body from head to toe may be treated. Also, the epidural space may be used for injection of local anesthetics to provide regional anesthesia required for various types of surgery. This is especially important when general anesthesia may be contraindicated due to a patient's condition or the particular surgery to be performed.
The epidural space is exceedingly narrow, is close to the spinal cord and, accordingly, accurate identification of the epidural space is exceedingly important. Unfortunately, because of the small size of the epidural space, it is often difficult to locate by even the most skilled physician.
One benchmark for locating the epidural space is the ligamentum-flavum. The ligamentum-flavum is a ligament having unique pressure characteristics which distinguish it from all other anatomical structures in the posterior spine. The ligamentum-flavum is useful in identifying and locating the epidural space because once the ligamentum-flavum is identified, the epidural space is easily identified - the epidural space lies only a few fractions of a centimeter anterior to the ligamentum-flavum.
As is well-known according to various anatomy handbooks, such as Gray's Anatomy, the spine has four major divisions--cervical, thorascic, lumbar and sacral. When probing the posterior back at a level below a spinous process, the major anatomical structures encountered when proceeding from dorsal to ventral are the skin, the supraspinous ligament, the intraspinous ligament, the ligamentum-flavum, the epidural space, the dura, the sub-arachnoid space containing spinal fluid and, finally, the spinal cord.
The main body of the spinal cord ends at about the beginning of the lumbar portion of the spine, around the vertabrae generally identified L.sub.1 -L.sub.2. Below this point the spinal cord divides into numerous small filaments, known as the tail of the horse or cauda-equina. The majority of epidural injections are performed at levels below lumbar vertabrae L.sub.2, to avoid the devastating consequences of an accidental spinal cord penetration. Because of this danger, many physicians and other clinicians shy away from performing upper level or cervical-thoracic epidural injections because confirmation of correct placement of the injecting needle in the epidural space is often difficult if not impossible using apparatus and methods known heretofore.
The ligamentum-flavum consists of very dense fibernous-membranous tissue, in contrast to the more spongia-type of tissue prevalent in other posterior spinal structures. Because of its unique composition and characteristics, the ligamentum-flavum (which may be the "benchmark" referred to hereinafter) is the only anatomical area in the posterior spine which resists externally applied positive pressure. In other words, air or fluid may be applied externally, against the ligamentum-flavum under a certain amount of positive pressure, whereupon the air or gas is resisted by this ligament, which does not readily transmit, permit diffusion of or propagate such air or gas pressure, in normal, healthy adult patients. In some patients the ligamentum-flavum is denser than in other patients. In patients with less dense ligamentum-flavums, particularly in elderly people in which the integrity of the ligamentum-flavum tissue may have degraded, the ligamentum-flavum may permit slight diffusion of air or gas applied thereagainst. However, in normal, healthy adults, no such diffusion will occur at any conventional gas pressure used for therapeutic application. The ligamentum-flavum makes up the bulk of the posterior border of the epidural space (which may be the "target tissue" referred to hereinafter).